How to make mushrooms stand on floor?
This is because hair is grown from the origin center of the original mushroom i.e. origin center is in center of the hair object that is mushroom. Try changing the origin center from center to bottom of the mushroom you can do it just by placing 3d cursor at the bottom and then press "Shift + s" and then origin to 3d cursor,
Another way of doing this is Just select the mushroom and go to edit mode and in edit mode press 'A' to ensure All Vertices are selected and then move the mushroom up so that origin point comes to bottom and then just go back to object mode and this will do,

Overall you have to change the centre of the origin of object to it's bottom so that hair is grown from the bottom
i hope you understand